1. Field of the Invention
The present invention relates to a device for electrically grounding a float glass production apparatus comprising a float tank with a metal bath; a glass melt producing unit, which comprises a melting tank for making the glass melt and a refining tank for degassing the glass melt; conducting devices for conducting the glass melt from the glass melt producing unit to the metal bath in the float tank and, as required, auxiliary units.
2. Related Art
The production of float glass, especially of special glass, occurs in a so-called flat glass production apparatus of a flat glass plant. The term “float glass production apparatus or plant” in the sense of the present invention means the entire apparatus of a typical structure, i.e. a production unit, which includes the entire hot region from the melting tank and refining tank up to the float bath upstream of the annealing system. Since the hot glass is a good ion conductor, i.e. behaves like an electrolyte, the production steps of the “hot region” are unavoidable, also electrolytically connected with each other. Because of the typical structure of the production unit also part of the glass melt has at least locally direct conducting contact to the metal parts of this apparatus. These metal parts thus can be characterized as or have the character of electrodes, at least in an electrochemical sense. A short-circuited electrochemical battery is realized with each exterior low-resistance connection between each pair or several of these electrodes, e.g. by a common ground. The associated short-circuit current flows as a direct current through the electrolyte glass and can be the cause of bubble-forming or alloy-forming side reactions at the so-called interface, the region between the melt producing vessel and the float tank.
Examples of parts, which have direct contact with the glass and which have the potential to act as electrodes, include grounding electrodes, heated electrodes from Pt, Mo, Ir, etc., glass level meters (operating according to the principle of resistance measurement), direct thermocouple elements, Mo enclosure (for corrosion of the fire-resistant surrounding wall), bottom outlet, outlet pipes, stirrers and shut-off slide (also called “tweel”) made of platinum or its alloys with other noble metals and/or coated with these other metals, and the tin bath.
Besides these so-called “notorious” direct electrode types other potential electrodes exists, which indeed have no direct contact with the glass but in spite of that are electrochemically connected to the glass surface. That means all metallic structures with free surfaces in the superstructure of the melting tank and refining tank or of the troughs. Included among these parts are, among others, direct thermo-elements in arched structures, viewing flaps or lids, parts of the overflow, gas and oil burners and parts of the loading apparatus. The electrolytic contact of these indirect electrodes occurs in this case by means of the hot gas atmosphere. Electrochemical equilibrium between the free glass surface and the concerned metal parts is guaranteed when the gas atmosphere contains sufficient amounts of redox active chemical components. Mixtures of redox pairs, water/hydrogen, water/methane or CO2/CO, are examples of redox active chemical components. Appropriate boundary surface reactions and rapid transport of gaseous species in the atmosphere above the melt guarantee exchange and transmission of charge equivalents over large distances—the gas atmosphere thus has quasi-electrolytic properties.
A similar situation exists in a typically grounded float bath, the float tank. The grounding of the glass typically occurs via the liquid tin and the metallic housing. The forming gas atmosphere has direct, surface contact with both and it fulfills the requirements of a buffering redox system with its component mixture, water/hydrogen. Normally because of that it would be guaranteed that the platinum coated tweel at the entrance to the float tank is electrically connected with the float bath ground. The extent of the short circuit can be controlled by the local composition of the forming gas: high hydrogen content in the vicinity of the tweel promotes e.g. current flow from the melting tank to the float tank ground.
The huge difference in the concentration of oxygen in the melting tank and the float bath is the main situation that a direct voltage arises in the apparatus. Oxygen pressure of one to up to two bar must be produced in the refining tank with oxygen-refined special glass, in order to guarantee formation of refined bubbles. In contrast the oxygen pressure in the float bath must be 10−15 to 10−18 bar, to prevent coating of the tin bath with a tin oxide layer. A reduction of the concentration difference is thus not possible.
Thus a short circuit electrochemical chain arises so that direct currents flow from anode to cathode (positive current flow) in the glass. The interior resistance of this short-circuited battery is the sum of all partial resistances for charge transport and boundary surface conversion. The current strength is then the quotient of the battery voltage and the total resistance.
The current flow has consequences for                local bubble formation at all phase transitions between glass and metal, e.g. at joining points between outlet pipe segments made of Pt, at stirrer surfaces, at the tweel and at glass/Sn contacts;        local corrosion of Pt components at the interface because of increased incorporation of alloyable glass components, especially Sn, Si, . . . ;        the monitoring of the electrochemical potential of metallic components, special stirrers, outlet pipe segments or tweel. The direct currents in the measurement path, reference/component are always considered in evaluating electromotive forces, which are measured between the concerned component and a suitable reference electrode.        
The essential prerequisite for occurrence of direct current flow with disadvantageous effects is the existence of a sufficiently low-resistance connection between at least two potential electrodes. This sort of connection is realized intentionally or unintentionally by a so-called ground connection in the daily production routine.
Intentional ground conditions are frequently closely connected with regulations for guaranteeing personal safety. They replace laborious or elaborate safety measures for routine handling. Likewise frequently ground-free construction of an apparatus means an expensive selection of materials during design or it simply is not a useful solution.
If the principle of ground-free construction is not part of the specifications of the apparatus, there are consequences. Once built up, many ground connections may not be undone with reasonable effort or expense.
The most important example here is the tin bath in the float tank, whose subsequent ground disconnection is not possible. Generally the liquid tin in the completely hot region remains the single direct connection to ground, but all would still be in order from an electrochemical viewpoint, when the current circuit is not closed on account of the absent second and further ground connections.
However other grounds may exist based on that. The supply apparatus, the overflow and the burners in the melting tank come into question. Subsequent ground disconnection is indeed possible in principle but only with a great effort or expense. The electrical contact here occurs by means of the furnace atmosphere, i.e. an actual ground circuit formed is high resistance. Typically also the ground is not definitely localized and it is characterized as a virtual ground.
Usually all measuring units (thermocouple elements, glass level meters), viewing closures, stirrers and all components at the interface, which are made of platinum or its alloys with other noble metals, are built-in without grounding without further effort.
Every noble metal uncoated tweel is designed “weakly” grounded by the forming gas atmosphere (i.e. a high resistance ground).
Additional grounds may exist at the so-called cold end, i.e. at the cooling sheet; however they are all not damaging from an electrochemical viewpoint and thus must be not considered further.